The present invention is directed to ink compositions and printing processes. More specifically, the present invention is directed to ink compositions containing liposomes of a vesicle-forming lipid and a dye, and to printing processes employing said inks. One embodiment of the present invention is directed to an ink composition which comprises an aqueous liquid vehicle, a dye, and a vesicle-forming lipid, wherein vesicles of the lipid are present in the ink.
Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, they are much simpler than the continuous stream type. There are two types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
The second type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink-filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280.degree. C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses on the resistor. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction towards a recording medium. The resistive layer encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet processes are well known and are described, for example, in U.S. Pat. No. 4,601,777, U.S. Pat. No. 4,251,824, U.S. Pat. No. 4,410,899, U.S. Pat. No. 4,412,224, and U.S. Pat. No. 4,532,530, the disclosures of each of which are totally incorporated herein by reference.
Known ink jet inks generally comprise a water soluble dye which is soluble in an ink vehicle such as water or a mixture comprising water and a water soluble or water miscible organic solvent. Inks comprising soluble dyes may exhibit many problems, such as poor waterfastness, poor lightfastness, clogging of the jetting channels as a result of solvent evaporation and changes in the solubility of the dye, dye crystallization, ink bleeding when prints are formed on plain papers, poor thermal stability, chemical instability, ease of oxidation, and low drop velocity. In addition, many of the dyes contained in inks may be potentially toxic or mutagenic. These problems can be minimized by replacing the dyes used in ink formulations with insoluble pigments. In general, pigments are superior to dyes with respect to waterfastness, lightfastness, image density, thermal stability, oxidative stability, compatibility with both coated/treated and plain papers, image edge acuity, reduced image feathering, and non-toxic and non-mutagenic properties.
Heterophase inks containing pigment particles as colorants, however, also exhibit difficulties. For example, the particulate colorant may exhibit a tendency to settle out or separate from the liquid vehicle, particularly when the ink is stored for long periods of time. In addition, inks containing pigment particles as colorants tend to be opaque instead of transparent, which reduces their usefulness for printing images on transparencies for the purpose of overhead projection. Further, inks containing pigment particles as colorants tend to clog the narrow orifices of the printhead, resulting in deterioration of the print quality.
U.S. Pat. No. 4,877,451 (Winnik et al.), the disclosure of which is totally incorporated herein by reference, discloses ink jet ink compositions comprising water, a solvent, and a plurality of colored particles comprising hydrophilic porous silica particles to the surfaces of which dyes are covalently bonded through silane coupling agents. In addition, U.S. Pat. No. 5,378,574 (Winnik et al.), the disclosure of which is totally incorporated herein by reference, discloses ink jet inks and liquid developers containing colored particles comprising hydrophilic porous silica particles to the surfaces of which dyes are covalently bonded through silane coupling agents.
U.S. Pat. No. 5,145,518 (Winnik et al.), the disclosure of which is totally incorporated herein by reference, discloses an ink composition which comprises an aqueous liquid vehicle and particles of an average diameter of 100 nanometers or less which comprise micelles of block copolymers of the formula ABA, wherein A represents a hydrophilic segment and B represents a hydrophobic segment, and wherein dye molecules are covalently attached to the micelles. Optionally, silica is precipitated within the micelles. In a specific embodiment, the dye molecules are substantially colorless and the dye is detectable when exposed to radiation outside the visible wavelength range. In another specific embodiment, the ink also contains a colorant detectable in the visible wavelength range.
U.S. Pat. No. 4,880,432 (Egan et al.), the disclosure of which is totally incorporated herein by reference, discloses a process for preparing particles colored with a dye which comprises: (a) forming, by a free radical dispersion polymerization process in a nonaqueous solution, polymeric particles having attached thereto stabilizing copolymers with at least one functional group capable of undergoing a chemical reaction with a dye, the particles having an average diameter of from about 0.1 to about 20 microns; (b) adding a dye to the polymeric particles having attached thereto stabilizing copolymers; and (c) effecting a chemical reaction between the dye and the stabilizing copolymers that results in the dye becoming covalently bound to the polymeric particles. Also disclosed is a liquid electrophotographic developer composition comprising a liquid medium, a charge control agent, and colored polymeric toner particles prepared as stated above and having an average diameter of from about 0.5 to about 5 microns.
Copending application U.S. Ser. No. 08/567,637, filed concurrently herewith, entitled "Ink Compositions with Liposomes Containing Photochromic Compounds," with the named inventors Carol A. Jennings, Marcel P. Breton, Mary A. Isabella, Eric G. Johnson, Trevor I. Martin, and John F. Oliver, the disclosure of which is totally incorporated herein by reference, discloses an ink composition which comprises an aqueous liquid vehicle, a photochromic material, and a vesicle-forming lipid, wherein vesicles of the lipid are present in the ink.
Although known compositions are suitable for their intended purposes, a need remains for ink compositions exhibiting advantages of both dye-based inks and pigment-based inks. There is also a need for ink compositions with good waterfastness characteristics. A need also remains for ink compositions exhibiting good lightfastness characteristics. Further, there is a need for ink compositions that are non-toxic and non-mutagenic. In addition, a need exists for ink compositions for which a wide variety of color choices exists. There is also a need for ink compositions suitable for printing on plain papers, coated or treated papers, and transparency materials. A need also remains for ink compositions with good thermal and oxidative stability. Further, there is a need for ink compositions suitable for ink jet printing that jet well and do not induce clogging of the printhead. A need also remains for ink compositions that when printed on substrates exhibit no undesirable intercolor bleeding between areas of different color. In addition, there is a need for ink compositions suitable for printing on hot substrates.